Ultrasonic, flow measuring devices are applied widely in process and automation technology. They permit simple determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the travel-time difference principle. In the travel-time difference principle, the different travel times of ultrasonic waves, especially ultrasonic pulses, so-called bursts, are evaluated relative to the flow direction of the liquid. For this, ultrasonic pulses are sent at a certain angle to the tube axis both with as well as also counter to the flow. From the travel-time difference, the flow velocity and therewith, in the case of known diameter of the pipeline section, the volume flow can be determined.
The ultrasonic waves are produced, respectively received, using so-called ultrasonic transducers. For this, ultrasonic transducers are secured in the tube wall of the relevant pipeline section. There are also clamp on-ultrasonic, flow measuring systems. In the case of these systems, the ultrasonic transducers are pressed externally on the tube, or pipe, wall. A great advantage of clamp-on, ultrasonic, flow measuring systems is that they do not contact the measured medium and can be placed on an already existing pipeline.
Ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. The ultrasonic waves are produced in the electromechanical transducer element as acoustic signals and, in the case of clamp-on systems, led via the coupling layer to the tube wall and from there into the liquid, or, in the case of inline systems, they are in-coupled via the coupling layer into the measured medium. The coupling layer is also, not so frequently, called the membrane.
Arranged between the piezoelectric element and the coupling layer can also be another coupling layer, a so called adapting, or matching, layer. The adapting, or matching, layer performs, in such case, simultaneously the function of transmitting the ultrasonic signal and reducing reflection at interfaces between two materials caused by different acoustic impedances.
Both in the case of clamp-on systems as well as also in the case of inline systems, the ultrasonic transducers are arranged on the measuring tube in a shared plane, either on oppositely lying sides of the measuring tube, in which case the acoustic signal extends, projected on a tube cross section, once along a secant through the measuring tube, or on the same side of the measuring tube, in which case the acoustic signal is reflected on the oppositely lying side of the measuring tube, whereby the acoustic signal traverses twice through the measuring tube along the secant projected on the cross section through the measuring tube. U.S. Pat. Nos. 4,103,551 and 4,610,167 disclose ultrasonic, flow measuring devices with reflections on reflection surfaces provided therefor in the measuring tube. Also multipath systems are known, which have a number of ultrasonic transducer pairs, each of which forms a signal path, along which the acoustic signals travel through the measuring tube. The respective signal paths and the associated ultrasonic transducers lie, in such case, in mutually parallel planes and parallel to the measuring tube axis. U.S. Pat. Nos. 4,024,760 and 7,706,986 show, by way of example, such multipath systems. An advantage of multipath systems is that they measure the profile of the flow of the measured medium in the measuring tube at a number of locations and can thereby provide highly accurate measured values for the flow. This is achieved, among other things, also by weighting the individual travel times along the different signal paths differently. Disadvantageous in the case of multipath systems are, however, their manufacturing costs, since a larger number of ultrasonic transducers and, in given cases, a complex evaluating electronics need to be installed.
Different proposals have been made for weighting the signal paths. The paper, “Comparison of integration methods for multipath acoustic discharge measurements” by T. Tresch, T. Staubli and P. Gruber, in the handout for the 6th International Conference on Innovation in Hydraulic Efficiency Measurements, 30 Jul.-1 Aug., 2006, in Portland, Oreg., USA, compares established methods for weighting the travel times along different signal paths for calculating flow.
German patents DE 198 61 073 A1 and DE 297 19 730 U1 each disclose a flow measuring system having a first sound path, which is reflected a number of times in the measuring tube.
U.S. Pat. No. 7,845,240 and European Patent, EP 2 282 178 A1, each disclose a flow measuring device, which, starting from a transmitter, transmits a first signal path, which sends a signal via a double refraction a receiver. Then, the receiver becomes a transmitter and sends an ultrasonic signal on a second signal path by means of a double reflection or multireflection back to the original transmitter, which has in the meantime become a receiver. This measuring arrangement includes a signal evaluation, which takes into consideration the values of the first and second signal paths. Disadvantageous in this situation is that during the course of traversing the first and second signal paths the properties of the flow have already changed, so that, for example, a rotation of the medium in the measuring tube is not taken into consideration, since it is registered only in one direction, however, not in the opposing direction.
European Patent, EP 0715 155 A1 has a measuring arrangement with multiple refraction, wherein the subsections of the signal path form only one plane, which extends parallel to the measuring tube axis. Because of this, for example, a rotation of the medium in the pipe cannot be compensated.
Published International Patent Application, WO 02/44662 A1 discloses a flow measuring device, in the case of which a signal is guided by multireflection on a signal path through a measuring tube. In such case, the path portions of the signal path form a single plane, which extends parallel to the measuring tube axis. Also here, for example, no compensation of rotation can occur. In such case, indeed, subsections of a signal path extend on a shared plane, but these subsections do not follow directly one after the other.
German Patent, DE 298 03 912 U1 shows in FIG. 2 a signal path with two directly sequential and therewith corresponding subsections 10 and 11, which lie on the same plane. Other directly mutually corresponding subsections, which lie on a shared plane, are not disclosed in this publication, since the two subsections 5 and 14 do not correspond with one another.
Published International Patent Application, WO 2010/002432 A1 has an arrangement of transducers, however, no reflection surfaces.
Published International Patent Application, WO 1995012110 A1 shows an ultrasonic, flow measuring device with a measuring tube with planar walls and a straight measuring tube axis and at least one reflection surface in the measuring tube with a surface normal on this reflection surface, which has three non-zero components in a right angled coordinate system, whose one axis corresponds to the measuring tube axis. The document teaches that an ultrasonic signal with a predetermined width, which is clearly greater than a dot shaped signal, has a Gauss shaped sensitivity distributed over this width and is used for flow measurement. The width of the signal corresponds, in such case, approximately to the width of the rectangular measuring tube. If such a signal would now extend through the measuring tube parallel to the side walls, the region with the highest sensitivity would extend through the middle region of the measuring tube, and, thus, also record the higher flow velocities with higher values. In the case of very small flow velocities, this would lead to measurement error. The document teaches, consequently, further, to irradiate the measuring tube largely homogeneously by leading the ultrasonic signals through all regions of the measuring tube by means of oriented reflections. For illustration, the broad ultrasonic signal was represented by individual beam portions. The path lengths of the individual beam portions are equally long, so that the beam portions do not cancel by interference.